Post and rail construction is widely used in a plethora of applications, including fencing, framing and scaffolding. The fasteners used for securing the generally vertical posts to generally horizontal rails have for the most part been the traditional coach bolt and nut, threaded rods with a nut both ends and other well known fixings.
In the art of post and rail fencing, it is known that wire tying, notching and strapping may be used in addition to the abovementioned prior art fixings.
Where coach bolts or threaded rods are proposed to be used for post and rail construction, the post and/or the rail is generally checked, notched or recessed to ensure positive location of the two members. The post and the rail are offered up or aligned and then drilled to provide a continuous hole for passage of the bolt or rod. The bolt or rod is then inserted through the hole to effect a joint between the post and rail. Subsequently washer(s) are fitted as required.
A disadvantage of this form of construction includes the protrusion of the bolt or threaded rod on both sides of the joint. In the case where threaded rods are used this protrusion is of a quite hazardous nature, the exposed threaded end of the rod being capable of inflicting injury to both personnel and livestock.
A second disadvantage lies in the requirement for a precise diameter hole to be drilled, large enough to ensure passage of the shank of the bolt, but not so large that the joint is sloppy.
Where wire tying is proposed, the last mentioned disadvantage is substantially overcome. However wire tying is labour intensive, involving the procedure of providing at least two holes drilled through both members, threading the tie wire through the holes and around a keeper and then crimping the disengaged ends under tension. The wire tied joint is also prone to loosening over time; since there is no provision for retightening a wire tie, the ongoing maintenance involves periodic replacement of the ties with the attendant high labour cost.
Loosening of the joint is also a problem for top rails secured to the top of posts by strapping. The strapping is generally affixed to opposed locations of the post by nailing. Although strapping is quick to instal and does not require the drilling of holes it has the added disadvantages of hazard wherein after a period of time the strap edge may extend or turn outwardly with the ancillary effect of nail loosening. It is also impossible to tension such strapping, resulting in a loose rail. To obtain the best possible result in strapping a top rail, it is desirable to V-notch the top of the post to accept the rail, which procedure is time consuming.
Upon a review of relevant prior art, it is clear that it is well known to provide barbs or protrusions on shanks or fasteners to resist withdrawal of the fastener from a workpiece. Thus for example in U.S. Pat. No. 4,653,486 there is described a fastener particularly suited for orthopedic use having a head, a tip, and a shank having a thread bearing zone and a thread free zone. The thread bearing zone includes a plurality of spaced barbs forming a discontinuous screw thread. Each protrusion in one embodiment was of uniform height not possessing a clearly defined leading edge and trailing edge and in another embodiment each protrusion had a leading arcuate surface again not possessing a clearly defined leading edge and trailing edge. One disadvantage of the particular fastener if used in post and rail construction was that it possessed limited holding strength by virtue of the fact that it would cut its own withdrawal track when forced into a wooden substrate and thus was relatively easily withdrawable. This fastener was also not appropriate for use with predrilled holes.
Similar comments could be made in relation to U.S. Pat. No. 3,983,779 which describes a sheet metal nail having a heading protrusion of pyramidal shape having a pointed vertex which provided a lead hole for subsequently located locking tangs on a shank of the rail. The locking tangs were punched from the metal sheet and included a leading portion and trailing portion offset to each other.
Again disadvantages as described above in relation to U.S. Pat. No. 4,653,486 and U.S. Pat. No. 3,983,779 were also applicable to other prior art which included West German specification 3335702 which included barbs orientated in a direction parallel to the longitudinal axis of the fastener, U.S. Pat. No. 3,850,073, U.S. Pat. No. 3,987,698, French Patent 2314318, U.S. Pat. No. 4,697,045, U.S. Pat. No. 4,395,174 and West German Specification 3836074. Another problem with these prior disclosures was that generally they referred to very specific applications which necessitated appropriate structural characteristics of the fastener dependent upon the precise application and as a result the fastener had little holding power when driven into substrates contemplated by the present invention. Thus for example U.S. Pat. No. 4,395,174 was directed to anchoring a sheet metal roofing panel to a roof support sheet metal beam, U.S. Pat. No. 4,653,486 discussed above was directed to a fastener for orthopedic use, U.S. Pat. No. 3,850,073 was directed to attachment of roofing insulation to a sheet metal base and U.S. Pat. No. 3,987,698, directed to a nail having a series of barbs in the form of wedge like ramps and having a smaller end directed at the tip or point of the nail was also directed to a specialized function totally unsuitable to insertion in predrilled holes in post and rail construction.
Reference may also be directed to U.K. Specification 2218061, U.K. Specification 1380500, U.S. Pat. No. 4,014,244, USSR Reference 842249, U.S. Pat. No. 4,454,875, U.S. Pat. No. 4,402,641, U.S. Pat. No. 4,233,878, U.S. Pat. No. 3,882,755, EP Specification 164432-A, West German Specification 3620372, West German Specification 2038885, USSR Specification 566981, West Germany Specification 3900870, Swedish Specification 8700352, French Specification 2582754, U.S. Pat. No. 4,718,802, French Specification 2570140, French Specification 2503290, Swedish Specification 8100909, West German Specification 3029923, West German Specification 3014745, West German Specification 3107403, U K. Specification 2063350, U.K. Specification 2021227, U.K. Specification 1442077 and West German Specification 2453891 as being background prior art not particularly relevant to the present invention.